Ever since the discovery of polytetrafluoroethylene (PTFE) polymer researchers have been looking for ways to reduce the particle size of as polymerized PTFE resin, comminuting produced anisotropic particles which required equipment pressures above 2,000 psi that have severely limited the resins capability. Products produced contained pores and exhibited poor properties that severely restrict PTFE resin utility. By the time the cause for these limitations was fully recognized the Plunkett patent (U.S. Pat. No. 2,230,654) had expired (1958). No further research dollars were spent on “TEFLON” PTFE resin, interest shifted to melt processable “TEFLON”: tetrafluoroethylenehexafluoropropylene (FEP), perfluoroalkoxy tetrafluoroethylene (PFA), and ethylenetrifluoroethylene (ETFE), etc. to solve PTFE's problems.
Advanced technology practiced in this invention looks at methods beyond early polymer chemistry to solve PTFE resin fabrication problems. The simple steps of attrition, temperature and pressure used in the art for low molecular weight polymers like polyethylene and nylon do not apply to PTFE resin's problems.
Early recognition of PTFE polymer's high molecular weight and high crystalline content, near 100 percent, set the stage for research. Serious research didn't begin until the early 1950's, twelve years after the PTFE patent (U.S. Pat. No. 2,230,654) was filed, interrupted by World War II. Because the problem seemed too complex for a quick solution, the industry dealt with the problem by developing lower molecular weight melt processable resins FEP, PFA, ETFE, and others in hope of solving PTFE's problems. FEP was introduced coincident with the decision to stop PTFE research.
It became evident early that the large polymerized PTFE resin aggregates would require attrition later found to be the root cause for poor moldability and also that water cut coagulated dispersion polymer by Thomas (U.S. Pat. No. 2,936,301) had the same strain instability characteristics; both particle forms were anisotropic with built in memory and a desire to relieve strain when compressed or sintered.
Unlike natural fibers and synthetic fibers of commerce, the fibers of this invention are isotropic and remain form stable with changes in temperature including the drying and final sintered steps. All synthetic melt processable fibers are melt-extruded and become anisotropic; consequently they shrink when exposed to high levels of heat because longitudinal orientation has been introduced by melt extrusion plus intentional fiber draw down introduced during cooling
All granular PTFE resins of commerce are anisotropic (Encyclopedia of Polymer Science and Technology (1970)). Granular PTFE resin is polymerized into large particles that must be reduced in size by attrition to be of any use. Attrition introduces molecular strain and anisotropy. Granular PTFE resin is used today predominantly in automatic compression molding where high preform pressures of the order of 5,000 to 10,000 psi are needed. Such pressure is far too high to be economical or practical if used for molding large parts. Small parts are used for mechanical friction and wear applications where part porosity is not important, like small diameter washers, rings, and sleeves.
The anisotropic fibers of Harford (U.S. Pat. No. 3,003,912) and Thomas (U.S. Pat. No. 2,936,301) and the isotropic product of this invention are possibly the only stand alone PTFE fibers available. The fibrils produced by W.L. Gore Company (Newark, Del.), Donaldson Company (Minneapolis, Minn.), and BHA Group (Atlanta, Ga.) are anisotropic and are an integral part of their product and an irreversible part of the finished sintered product, and all have limited utility in filtering and textiles products.
Anisotropic fibrils are prepared by Friedrich (U.S. Pat. No. 6,352,660), Debe (U.S. Pat. No. 5,910,378), Solomon (U.S. Pat. No. 4,339,325), and Ree (U.S. Pat. No. 4,153,661) but all are highly contaminated with foreign substances mixed or embedded in the product added to induce fibrillation or fiber-like structures that form during long batch processing working the ingredients abrasively in a viscous concentrated mix of hot molten polymer to become embedded into these particulates to contaminate and making it too difficult and costly to extract. All products produced using the methods of the Friedrich, Debe, Solomon, and Ree patents are anisotropic and are used unsintered to instill product porosity where contamination is not critical and where fusion temperature will not be encountered. The products have little strength.
The use of a contaminated fluoropolymer in products and/or processes is rare and practically unheard of. However, the use of contaminated fluoropolymer in chlor-alkali cells is a rare exception as these products have very limited use outside this industry.